1. Field of the Invention
The present invention generally relates to a laser beam scanning device which scans a surface to be scanned by a laser beam emitted from a light source, an image forming apparatus using the laser beam scanning device, and a laser beam detecting method by the laser beam scanning device.
2. Description of the Related Art
Recently, in an image forming apparatus of an electrophotographic system which is applied to a laser printer, a laser plotter, a laser facsimile apparatus, a digital copying apparatus, and so on, a color image is formed, and a high quality is required for the color image. In order to achieve the high quality of the color image, out of color registration must be decreased. The out of color registration can be decreased by applying several adjusting processes to the apparatus when the image forming apparatus is delivered from a factory to a customer. However, the out of color registration may be increased with the passage of time after delivering the apparatus.
There are two kinds of out of color registration, that is, out of color registration in the main scanning direction and in the sub main scanning direction.
The out of color registration in the sub scanning direction with the passage of time may be caused by a positional shift of an optical element such as a lens and a mirror and deformation of a housing in which a light source and the optical elements are disposed. When a positional shift of a light spot formed on the surface of a photoconductor drum in the sub scanning direction is detected, the out of color registration in the sub scanning direction can be corrected by a suitable method.
In Patent Document 1, the out of color registration in the sub scanning direction is corrected by using a liquid crystal deflection element. That is, a laser beam is deflected in the sub scanning direction by using the liquid crystal deflection element. In Patent Document 2, the out of color registration in the sub scanning direction is corrected by using a wedge-shaped prism. That is, a laser beam is deflected in the sub scanning direction by rotating the wedge-shaped prism around the optical axis.
In Patent Documents 3 and 4, a positional shift of an optical spot in the sub scanning direction is detected by using a photodiode having non-parallel sides. That is, the positional shift of the optical spot in the sub scanning direction is detected by using a time interval between two signals output from the two sides of the photodiode.
In addition, as a color image forming apparatus, there is a tandem type color image forming apparatus. In the tandem type color image forming apparatus, plural image carriers such as photoconductor drums are disposed, and a laser beam scanning device scans the surface of each image carrier by using a corresponding laser beam. The laser beam is output from a semiconductor laser which is driven by the corresponding color image information signal.
Each laser beam is condensed on the surface of the corresponding image carrier which is uniformly charged, via a deflection scanning unit formed of, for example, a polygon mirror, and an optical component such as a condenser lens, and the laser beam scans the surface of the image carrier in the main scanning direction due to the deflection scanning unit.
Together with the scanning of the laser beam in the main scanning direction, each image carrier is rotated around its axle, and an electrostatic latent image corresponding to an image signal is formed on the surface of each image carrier by laser beams having a predetermined interval in the sub scanning direction. The electrostatic latent image on the surface of each image carrier is developed by corresponding color toner, and a full color image is formed by superposing color toner images on a recording medium (paper).
In the laser beam scanning device which is used in the tandem type color image forming apparatus, laser beams scanning on the surfaces of the image carriers have different routes among different colors. When the condenser lens is deformed by heat indicated by an environmental temperature rise and a temperature rise inside the tandem type color image forming apparatus, the position of the scanning laser beam is likely to be changed. Especially, when a resin condenser lens is used, since the thermal expansion coefficient of the resin condenser lens is large, the positional change of the scanning laser beam becomes large due to the temperature change.
In the tandem type color image forming apparatus, when an image writing start position by scanning of each laser beam is shifted, since a color image is formed by superposing different color images, image quality is degraded by the out of color registration of transferred different colors. In order to avoid the above problem, a light receiving element is disposed so that each laser beam can be individually detected at the deflection scanning start position, and a time from an output of a detection signal to a writing start is adjusted in each laser beam. With this, the image writing start positions of plural laser beams conform to each other. The image writing start positions are adjusted in the main scanning direction.
The out of color registration also occurs in the sub scanning direction orthogonal to the main scanning direction. The out of color registration in the sub scanning direction occurs due to various reasons such as an optical axis shift of the laser beam by a temperature change and eccentricity of an image carrier, for example, a photoconductor drum.
In order to avoid the above problem, by using an out of color registration detecting pattern formed on a transfer body, a scanning positional shift is periodically detected at the start-up time of the apparatus or at the time between jobs, and the scanning positional shift is corrected. However, when printing is continuously executed, the scanning positional shift is increased by heat from a fixing unit and a polygon mirror motor; therefore, when the number of printing cycles of one job is large, the out of color registration is gradually increased.
In Patent Document 5, a laser beam scanning device and an image forming apparatus using the laser beam scanning device are disclosed. The image forming apparatus provides a polygon mirror, a light receiving element, a controlling unit, and an optical system. The polygon mirror deflects and scans plural laser beams emitted from plural semiconductor lasers by deflecting and reflecting. The light receiving element receives the laser beams deflected and scanned by the polygon mirror by inputting from one side orthogonal to the scanning direction and outputs the received laser beams from the other side inclining to the scanning direction. The controlling unit makes the semiconductor laser emit laser beams corresponding to image information based on a signal output from the light receiving element receiving the laser beams. The optical system leads the laser beams emitted from the semiconductor laser and reflected from the polygon mirror to plural photoconductor drums.
In the laser beam scanning device and the image forming apparatus using the laser beam scanning device, when a laser beam is shifted in the sub scanning direction, timing is not changed at which timing the light receiving element detects the laser beam in the input side of the scanning direction of the light receiving element; however, timing is changed at which timing the light receiving element detects the laser beam in the output side of the scanning direction of the light receiving element. Therefore, the shifting amount of the scanning laser beam in the sub scanning direction can be calculated based on the timing shift therebetween. When a correcting unit for correcting the calculated shifting amount is used, the shifting amount can be corrected.
In Patent Document 6, an image forming apparatus which can detect a pitch shift between plural laser beams in the sub scanning direction is disclosed. In the image forming apparatus, in order to detect laser beam positions, plural sensors composed of light receiving elements whose scanning start point sides in a laser beam detecting region are not parallel to each other are disposed. A time interval between laser beams which pass through the scanning start points of the plural sensors is calculated by individually turning on the plural laser beams, and the difference between the time intervals is converted into the sub scanning direction pitch between the laser beams.
However, in Patent Documents 5 and 6, when a positional shift amount of laser beams is detected by using a special-shaped light receiving element; the size of the light receiving element becomes large and the shape thereof becomes complex. Consequently, the layout of the apparatus becomes complex and the apparatus becomes large and these result in a cost increase.
[Patent Document 1] Japanese Laid-Open Patent Application No. 2003-241130
[Patent Document 2] Japanese Laid-Open Patent Application No. 2004-109700
[Patent Document 3] Japanese Laid-Open Patent Application No. 2005-37575
[Patent Document 4] Japanese Laid-Open Patent Application No. 2005-62597
[Patent Document 5] Japanese Laid-Open Patent Application No. 10-235928
[Patent Document 6] Japanese Laid-Open Patent Application No. 7-72399
However, in Patent Documents 3 and 4, when the special photodiodes are used, the cost is increased. Generally, the size of the special photodiode is larger than that of a general-purpose photodiode and the photodiodes are generally disposed outside an image forming region. That is, a region where a light deflector can scan must be large; as a result, the scanning optical system becomes large. When the scanning optical system becomes large, the optical path length becomes long. Consequently, it becomes difficult to manufacture the apparatus due to an increase of a sensitivity error and a large thickness of a scanning lens. That is, it is difficult to miniaturize the apparatus.
In addition, in order to detect a scanning positional shift by a laser beam, an exclusive special photo IC is disposed in the laser beam scanning device. The exclusive special photo IC includes a light receiving element and a comparator circuit; this causes high cost. When a general-purpose low-cost photo IC is used which IC has a simple-shaped light receiving surface, the scanning angle of a light deflector is narrow and the scanning positional shift of the laser beam can be detected outside the image forming region in the scanning range. Therefore, when the exclusive special photo IC is used, optical characteristics (fθ characteristics) of the scanning laser beams in the detecting region become largely different from those in the image forming region; consequently, detecting accuracy becomes low.
In addition, when the general-purpose low-cost photo IC is used, detection accuracy is degraded caused by various factors relating to a change of the light receiving amount. The change of the light receiving amount is caused by an output change of a light source caused by, for example, a temperature change, deterioration of reflectance and transmittance of an optical element with the passage of time, and a change of the rotational speed of the light deflector corresponding to a pixel density change at the time of image forming.